Direct field effect

Three aspects of the inductive effect have to be considered the inductive effect, the inducto-electromeric or rr-inductive effect, and the direct field effect. The first of these is the one most frequently... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

Earlier in this book ( 7.2.1) the direct field effect was mentioned in rather general terms. The foregoing discussion brings out the fact that there are three aspects of the effect which have to be considered ... 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (z) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the TT-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same 226... 

Meyers212 collected together several examples of the anomalous behavior of sulfones in which the group was close to the reaction center and interpreted these in terms of the negative direct field effect of the S02 oxygen atoms (cf. Meyers and colleagues141). 

The greater sensitivity of the alkaline reaction to changes in substituent is readily explained in this way —the comparison of cis and trims compounds isolates not only the steric effect but also the direct field effect —but the correlation is poor with steric effects in this system. 

It is generally recognised that the inductive effect should be subdivided into a polarisation effect on the o-bond framework and also on the rr-electrons, and it has been indicated that a major effect of strongly electron-withdrawing groups like perfluoroalkyl is by a through-space polarisation of the aromatic rr-electrons (direct field effect). 

The influence of the direct field effect on the hyperfine structure is considered in more detail in the following section. Some technical properties are summarized in Appendix A4.1. 

In conclusion hyperfine structure affected by direct field effects are difficult to interpret because of the admixture of nuclear-spin states leading to the appearance of a complicated pattern of hyperfine lines. Systems featuring such effects are in general better studied by ENDOR. The effect can, however, be employed to determine the relative signs of the principal values of hfc tensors particularly for H and nuclei in cases where the ENDOR method is not applicable or is not available. The subject is discussed in the following section. 

E4.7 Suggest a reason why the direct field effect causes complications in ESR but not in ENDOR. 

The diamagnetic or complexation shift is usually small and is often neglected. It originates from effects such as conformational changes, inductive effects, and direct field effects. In satiuated ligands, the diamagnetic shifts are usually insignificant, with exception of the nuclei directly coordinated to the Lu + cation. ... 

Actually, the reaction may be influenced by environmental effects which are comparable with gas phase solvation by further infiltrated molecules. Furthermore, a direct (field effects) and/or indirect (mediated by the associates and other adsorbates) interaction of the adsorbat with other surface centres will occur. This can theoretically be treated and interpreted on the basis of an extended supermolecule approach. 

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